2 Answers

1 vote

Go to route 7 and go to the house east of celestial tower. There is an old man in there and he will register them in your pokedex. If you've played B/W, then it is the house that you originally find Tornadus/thundurus in front of before they run off and roam Unova
Source: strategy guide and http://bulbapedia.bulbagarden.net/wiki/Unova_Route_7

You trade them from Black and White or you capture one on Pokemon Dream Radar.You can ask for them in the GTS if go and talk to the man in the cabin east of celestial tower on route 7.He will register them in your pokedex.